1. Field of the Invention
The present invention relates to a four-stage type monochromator whose wavelength resolving power is improved with a small number of component parts and which has a high dynamic range.
2. Description of the Related Art
FIG. 6 shows an example of a conventional four-stage type monochromator. Reference numerals 24a to 24d denote monochromator units, and a single pass monochromator unit is formed by an entrance slit plate, a plane diffraction grating, an exit slit plate, and two concave mirrors. The incident light from a light source 1 is made incident through a slit in an entrance slit plate 25a, is converted to parallel light by a concave mirror 26a, and is made incident on a plane diffraction grating 27a. Of the incident light, a specific wavelength component, which is determined by an angle of rotation of the plane diffraction grating 27a having an axis parallel to the grating as its axis of rotation, is made emergent toward a concave mirror 26b as emergent light.
The reflected light from the concave mirror mirror 26b is made emergent through a slit in an exit slip plate 28a. The emergent light from the slit in the exit slit plate 28a is made incident on the double pass monochromator unit through a slit in an entrance slit plate 25b, is reflected by a concave mirror 26c toward a plane diffraction grating 27b, and is used as second diffracted emergent light. In the same way as the monochromator unit 24a, the plane diffraction grating 27b reflects the light at an angle which differs depending on the wavelength of the incident light, and a specific wavelength component, which is determined by the angle of rotation of the plane diffraction grating 27b having an axis parallel to the grating as its axis of rotation, is made emergent toward a concave mirror 26d as emergent light.
The reflected light from the concave mirror 26d is made emergent through a slit in an exit slit plate 28b, and is used as third-stage incident light. In the triple pass monochromator unit as well, diffraction similar to the second stage occurs, and its emergent light is used as incident light for fourth-stage incident light. In the fourth stage as well, diffraction occurs in a similar manner, and reflected light from a concave mirror 26h is made emergent from a slit in an exit slit plate 28d. 
In such a configuration, if the angles of the plane diffraction gratings 27a to 27d are set such that the wavelengths passing through the respective monochromator units 24a to 24d become equal, of the incident light from the light source 1, only a specific wavelength component can be led out through the slit in the exit slit plate 28d of the monochromator unit 24d. 
In such a configuration, the deviation of the angle of reflection at the wavelength occurring in the first diffraction becomes the deviation of the second incident light, and angular dispersion which is indicated by the angle of deviation of the angle of reflection per unit wavelength is added. As the angular dispersion is similarly added in the third and fourth diffraction as well, the amount of movement of an image per unit wavelength in the fourth-stage slit becomes large, so that it is possible to improve the wavelength resolving power at a position where the wavelength dropped by a predetermined level (3 dB) from a peak value of a mean wavelength xcex. In addition, the exit slit plates are provided for the respective monochromator units 24a to 24d, and since passing wavelengths are selected, it is possible to obtain a high dynamic range.
FIG. 7 shows another example of a conventional quadruple pass monochromator. This quadruple pass monochromator is comprised of an entrance slitplate, a plane diffraction grating, a concave mirror, and a plane mirror. The incident light from the light source is made incident through a slit in an entrance slit plate 29, is converted to parallel light by a concave mirror 30, and is made incident on a plane diffraction grating 31. Of the incident light, a specific wavelength component, which is determined by an angle of rotation of the plane diffraction grating 31 having an axis parallel to the grating as its axis of rotation, is made emergent toward a plane mirror 32 as emergent light. The emergent light is reflected by the plane mirror 32, and is made incident again on the plane diffraction grating 31. The second diffracted light from the plane diffraction grating 31 is reflected again by the concave mirror 30, and is made incident on a returning reflecting means 33.
The returning reflecting means in this example is formed by two plane mirrors and an intermediate slit plate, and emergent light from the returning reflecting means, from which only a specific wavelength component has been led out through a slit in an intermediate slit plate 36, is made incident on the concave mirror 30. The reflected light from the returning reflecting means is converted to parallel light by the concave mirror 30, and is made incident on the plane diffraction grating 31 as third incident light.
The third diffracted light from the plane diffraction grating 31 is reflected by the plane mirror 32, is made incident again on the plane diffraction grating 31 so as to be reflected as fourth diffracted light. The fourth diffracted light is reflected again by the concave mirror 30, and is focused onto a final slit plate 37 so as to select a passing wavelength.
In this example, since the wavelength of the emergent light is selected by causing the light to pass through two slits including the intermediate slit and the exit slit, it is possible to obtain a high dynamic range. However, although the angular dispersion which is indicated by the deviation of the angle of reflection per unit wavelength is added in the first and second diffraction, the angular dispersion is subtracted in the third and fourth diffraction in which the reflected light returning from the returning reflecting means is used as incident light. In the final slit, the dispersed light selected by the intermediate slit is recombined.
With the above-described first conventional example of the quadruple pass monochromator, four sets each comprised of two concave mirrors, two slit plates, and one plane diffraction grating which make up each monochromator unit are required, so that the number of component parts increases, and the size of the apparatus becomes large. In addition, since all the wavelengths of light transmitted through the monochromator units 24a to 24d must completely match, the plane diffraction gratings 27a to 27d must be rotated to such angles at which the light with the same wavelength is transmitted, a complicated synchronizing mechanism is necessary.
In addition, with the above-described second example of the quadruple pass monochromator, the deviation of the angle of reflection at the wavelength occurring in the first diffraction becomes the deviation of the second incident light, and the angular dispersion which is indicated by the angle of deviation of the angle of reflection per unit wavelength is added. However, in the third and fourth diffraction in which the reflected light returning from the returning reflecting means is used as incident light, since the deviation of the angle at the wavelength of the incident light is reversed, the angular dispersion of the emergent light from the diffraction grating is subtracted. In the final slit, the dispersed light selected by the intermediate slit is recombined, and in this aspect there has been a limit to the wavelength resolving power.
An object of the invention is to provide a four-stage type monochromator whose wavelength resolving power is improved with a small number of component parts and which has a high dynamic range.
To overcome the above-described problems, in accordance with the invention there is provided a four-stage type monochromator including a concave mirror for converting incident light to parallel light and for outputting the same; a plane diffraction grating for diffracting the parallel light; a plane mirror which has a reflecting surface substantially perpendicular to an optical path of the diffracted light diffracted by the plane diffraction grating and which reflects the diffracted light to cause the reflected light to be incident again on the plane diffraction grating; and returning reflecting means which returns the diffracted light which was diffracted again by the plane diffraction grating and was focused by the concave mirror, so as to cause the returned light to be incident again on the concave mirror, wherein the reflected light from the concave mirror is made incident upon the plane diffraction grating as third incident light, its diffracted light is reflected again by the plane mirror and is made incident on the plane diffraction grating as fourth incident light, so as to output the light diffracted by the plane diffraction grating and having a specific wavelength through a slit in an exit slit plate, characterized in that the returning reflecting means reverses a dispersing direction due to the wavelength of the light before and after its reflection.
By adopting the above-described arrangement, the diffraction grating diffracts the light four times in different dispersing directions, so that it becomes possible to obtain sufficient wavelength dissolving power with a small number of component parts. In addition, since the wavelength of the emergent light is selected by an intermediate slit and an exit slit provided in the returning reflecting means, it is possible to obtain a high dynamic range.